Add an option to scale down large images on iOS

Difference with the previous PR  #769:

- I have updated the scale down code, following the sample code of Apple "Large Image Downsizing"

Comment from the previous PR:
- I added an option to scale down large images as it is a common problem with SDWebImage.
- I chose to not add another method to SDWebImageManagerDelegate because I think we could choose the right maximum size for the library users.
Let me know what you think?



PS: I create a new pull request because I think my previous one closed can't be updated